(1) Field of the Invention
The present invention relates to a method of and an apparatus for controlling supercharge pressure for a turbocharger.
(2) Description of the Prior Art
A turbocharger is constructed in such a manner that an exhaust gas turbine is rotated by exhaust gas at high temperature and at high pressure so that the pressure within the intake manifold can increased above atmospheric pressure as the number of rotations or rotational speed of a compressor in the turbocharger increases. As a result, the supply of a large quantity of intake air flow to the engine becomes possible by the supercharge pressure thus obtained, with the result that high torque, high output power and improvement of fuel consumption can be obtained.
Now, in a car engine having a wide range of engine speeds, it is common to provide the supercharge pressure in the middle and high speed operating zones. In the low speed operating zones, however, as it is difficult to obtain a sufficient exhaust pressure, and the torque at low speed will tend to be insufficient, to provide the supercharge pressure. In this case, it is known that the determinant of the supercharge pressure in the lower speed operating zone is the ratio A/R where A is the cross sectional area of a scroll and R is the radius from the center of the scroll. Accordingly, if the cross sectional area can be made small in the low speed operating zone having a small quantity of the exhaust gas flow, the supercharge pressure can be increased by increasing the rotational speed of the turbine.
To this end, a turbocharger of the variable capacity type which has capacity changing means, with the ratio A/R of the turbine being variable, has already been proposed by the same applicant of this application (see, for instance, Japanese Patent Application No. 58-162918) in which a sufficient supercharge pressure can be obtained even when the turbocharger of the variable capacity type is operated in the low speed operating zone.
In the supercharge pressure control using the turbocharger described above, the provision is made for an actuator for driving the capacity changing means of the turbocharger, with the supercharge pressure being at work pressure or operation pressure which is produced downstream of the compressor and actual supercharge pressure is controlled at a target or preset supercharge pressure, i.e., the ratio A/R of the turbine is controlled so as to rapidly increase the actual supercharge pressure by controlling the duty value of an electromagnetic valve which discharges the operating pressure to outside.
In the control characteristic of an electromagnetic valve, where the X-axis indicates intake air flow rate and the Y-axis indicates a basic control duty value (see, for instance, FIG. 9(A)), the duty value signifies the opening time of the valve per a predetermined time. When the duty value is 100 percent it indicates that it is fully opened and the cross-sectional area A is made minimum in this case by means of the actuator and the capacity changing means, so as to increase the rotational speed of the exhaust gas turbine.
On the other hand, when the duty value is zero, it indicates that the electromagnetic valve is fully closed, with the result that the sectional area A is made maximum and the rotational speed of the turbine is suppressed, thus controlling the supercharge pressure to be rapidly increased and afterwards to be maintained constant. In the actual control, in this case, in order to overcome the deviation of control involving different dispersion factors, it is common practice to perform a feedback control in response to an actually detected value. In this example as well, the amount of the feedback correction is calculated from the deviation between actual supercharge pressure and a target or preset supercharge pressure, and the duty value is corrected by this amount.
The capacity changing means of the type described above is provided for the purpose of securing a sufficient supercharge pressure from the low speed and low load operating zone of the engine. In the high load operating zone, however, the rotational speed of the turbine becomes excessively high to control the supercharge pressure. Accordingly, it is a common practice to provide an exhaust bypass valve for bypassing the discharge pressure at a turbine inlet to the turbine downstream. However, the provision of such a plurality of control means for controlling the supercharge pressure as described will possibly result in unexpected disadvantages as a result of the interference of the two kinds of control. Namely, the capacity changing means and the exhaust bypass valve are both control means for the supercharge pressure so that the action of one means affects the other. For instance, when the capacity changing means deviates in the direction of closure, i.e., in the direction of increase of the supercharge pressure from an optimum value, the exhaust bypass valve will deviate in the direction of opening, i.e., in the direction of suppressing the supercharge pressure from the optimum value and the supercharge pressure is maintained at a preset value as a whole.
In this manner as described above, in order to achieve the purpose of maintaining the supercharge pressure at the preset value, there will be numerous approaches to realize the control of the capacity changing means and the exhaust bypass valve. However, in order to maintain the engine performance at its maximum, it is necessary that the control is switched in an optimum position as the capacity changing means and the exhaust bypass valve have an optimum position respectively, while maintaining the supercharge pressure at the predetermined value.
To this end, the operating zones for controlling the capacity changing means and the exhaust bypass valve may be set up without overlapping and a control may be changed-over from the capacity changing means or the movable tongue member to the exhaust bypass valve at a maximum capacity condition of the capacity changing means in accordance with the result of a decision for the operating zones.
However, when changing-over the control, in spite of the condition that the capacity changing means is set at the optimum position so that a maximum capacity thereof may be obtained, the movable tongue member, as the capacity changing means, is moved in the direction of closure of the guide path for the exhaust gas when the exhaust gas flow is increased after switching over the control. As a result, the capacity of the turbocharger tends to be lowered.
Namely, in FIG. 3, the movable tongue member 45 is normally mounted at the confluence of the end 41 of a scroll 39 and a guide path 40 to the scroll surrounding an exhaust gas turbine 37, which is gradually reduced toward downstream in the direction indicated by the arrow F.
With this structure, when the flow speed of the exhaust gas is increased, the flow speed of the gas in the guide path 40 is also increased, and this results in the pressure lowering in the exhaust gas. Consequently, the movable tongue member 45 is pulled in the direction of closure, i.e., to the left in FIG. 3, by the lowering of the pressure. As a result, the capacity of the turbocharger is increased, which also results in the lowering in the acceleration performance.